If you do not know, how to write the proper code in the websource script, ...
then you can code it afterwards into a Mp3tag action.
Action: Format value
Field: ARTIST
Formatstring: %ARTIST%$if2(', '%MIXARTIST%,)
... or ...
Formatstring: %ARTIST%[', '%MIXARTIST%]
... or ...
Formatstring: $trimRight(%ARTIST%,',')
See also ...
beatport.com scripts by stevehero (search & direct) - jaimz mod
beatport.com scripts by stevehero (search & direct) - jaimz mod
Need some help with IF
beatport.com scripts by stevehero (search & direct) - jaimz mod
beatport.com scripts by stevehero (search & direct) - jaimz mod
DD.20150304.1517.CET